February in the hills with Chris Horne and Barbara Mitcalfe

Hymenophyllum nephrophyllum, kidney fern, raurenga

This strikingly beautiful, endemic, filmy fern is one of New Zealand’s twenty-nine species in the family of filmy ferns, the Hymenophyllaceae.
Kidney fern is in the genus Hymenophyllaceae.
‘Hymeno’ comes from the Greek word for ‘membrane’, and ‘phyllum’ from the Greek word for ‘leaf’.
The first section of the second part of its name, ‘nephro’ comes from the Greek word for ‘kidney’, which aptly describes the shape of the shiny, green fronds.
Kidney fern, was formerly Cardiomanes reniforme, and then Trichomanes reniforme.

Look for it in the North Island, on the West Coast of the South Island, on Rakiura / Stewart Island, and on Rekohu / Chatham Island.
You may see it thriving in a wide range of habitats in lowland to montane forests, from damp forest on the West Coast, to humus on exposed lava fields on Rangitoto Island.

The glossy, translucent fronds are 3-10 x 4-13 cm, on stipes (stalks) 5-25 cm long. Have you noticed how the fronds curl up tightly in dry weather, to reduce moisture loss, then recover quickly after rain? Kidney fern creeps for many metres over the forest floor, sometimes forming extensive patches. It can climb banks and rock faces, and scramble up tree trunks.

The reproductive parts, spores, are contained in sori, which are crowded around the upper margin of the fronds, and slightly sunk into them. Each sorus is protected by a 'lid' of colourless issue called an indusium. When the spores are mature, the indusium shrivels, enabling the spores to be released.

Raurenga often grows in patches in which little else can grow. This is because it produces a phytotoxin which inhibits the root growth of other plants.

In New Zealand Ferns, published in 1921 by H B Dobbie, and revised by Marguerite Crookes in 1952, Dobbie described this plant’s unusual, un-fern-like appearance: “I shall never forget my first sight of it. I could not believe it was a fern.”